Various antennas have been used for mobile communication such as satellite communication, a global positioning system (GPS), and cellular phones. Thus, antennas are used for various purposes. Therefore, an increase of the bandwidth is required such that an antenna operates in a wide frequency band. Heretofore, antennas using a device that does not have a very wide band, such as slot antennas, have mainly been used for a parallel-plate transmission mode. However, various applications of a device having a wide band, such as a helical antenna, have been developed recently. Accordingly, a feeder circuit is also required to have a widened band.
There is an antenna developed by the inventors in order to widen the band of an antenna and a feeder circuit. FIG. 1A is a cross-sectional view of an antenna using a feeder circuit for a parallel-plate transmission mode, and FIG. 1B shows return loss characteristics of the antenna. The antenna 1 shown in FIG. 1A has an upper conductive plate 2, a lower conductive plate 3, a coaxial central conductor 4, a guide portion 5, and a short-circuit portion 7. The upper conductive plate 2 and the lower conductive plate 3 are provided substantially in parallel to each other. A central portion of the lower conductive plate 3 is recessed downward so as to form the short-circuit portion 7. A conductor at the bottom of the short-circuit portion 7 forms a short-circuit plate 8. The coaxial central conductor 4 protected by the guide portion 5 is fixed on the short-circuit plate 8 at a central portion of the antenna. In the present invention, the lower conductive plate 3, the coaxial central conductor 4, the guide portion 5, the short-circuit portion 7, and the short-circuit plate 8 of the antenna except the upper conductive plate 2 are collectively referred to as a feeder circuit.
In this antenna, the short-circuit portion 7, which is recessed downward at the central portion of the lower conductive plate 3, serves as an impedance conversion circuit, thereby increasing the bandwidth of frequencies. FIG. 1B shows the frequency dependency of return loss (RL) characteristics of this antenna. The return loss is defined by a ratio of an incident power to an antenna and a reflected power from the antenna. A small value of the return loss means that the antenna matches the frequency. In the present invention, if the return loss is equal to or smaller than −20 dB, i.e., if the loss of power is equal to or less than 1%, then it is determined that an antenna matches the frequency. Therefore, in the case of the antenna shown in FIG. 1, the central frequency is 7.75 GHz, the lower limit frequency is 7.4 GHz, and the upper limit frequency is 7.95 GHz. The bandwidth is 550 MHz, and the fractional bandwidth is 7.1%. The bandwidth of this antenna is wider as compared to conventional antennas and is improved to be 550 MHz. Nevertheless, there is a demand for further increasing the bandwidth of the antenna.